1. Field of the Invention
The present invention relates to a facsimile machine line control system. More particularly, the invention relates to a facsimile machine line control system that is suitable for overseas communications.
2. Description of the Related Art
Three types of network control unit (NCU) used for facsimile machines are: manual transmission--manual reception (MM) type; manual transmission--automatic reception (MA) type; and automatic transmission--automatic reception (AA) type. Within these types, the AA type, i.e., facsimile equipment with an automatic transmission function, has recently come to be the most common.
A facsimile machine with an automatic transmission function is one that can automatically call prespecified destinations and transmit image information at preset times. Generally, such a facsimile machine has an automatic redialing function to disconnect and call a station again after a preset delay if the facsimile machine receives a busy tone from the called station.
Busy tones are emitted by exchanges. An exchange detects a request from a calling station for connection to a called station, but if the called station is connected to another station at the time the exchange emits an audible signal, i.e., a busy tone. In the case of the Nippon Telegraph & Telephone Company (NTT), for example, 400 Hz signals (impulse ratio 60 IPM .+-.20%, make ratio 50.+-.10%) are transmitted as busy tones.
A facsimile machine or similar terminal device can detect such busy tones, cut the line, and then perform various types of processing such as redialing or, during sequential transfer or trunk broadcasts, etc., calling of subsequent stations and polling.
The following problem exists with line control systems which effect detection of busy tone and subsequent processing in this manner. Within Japan, for example, it is satisfactory if busy tones are detected and subsequent processing is performed by a facsimile machine line control system that has been designed for detection of busy tones in accordance with the above-noted NTT specification. However, in recent years direct connection with international circuits has become common, which means that there is an increasing need for detection of busy tones based on the exchange specifications of various overseas countries. However, conventional facsimile machine line control systems effect control based on domestic Japanese specifications, despite the fact that the busy tone specifications vary considerably in different countries.
There has been the problem that when such line control systems, which effect control based on domestic Japanese specifications, are used to conduct international communications, either they are incapable of detecting busy tones that are based on the independent specifications of individual countries or they are responsible for detection errors.
If a busy tone cannot be detected, it is necessary to wait for a set time for a reply before it is possible to come to the conclusion that a response from the called station is unobtainable. This means that until this conclusion is reached, wasted actions are performed because of the wait for a response and there is a delay in the starting of various forms of processing that are scheduled for execution after detection of a busy tone.